My first attempt was to construct a 500 kHz class D transmitter based on the DF3LP 136 kHz design and I failed miserably, whilst Dave G3WCB was having great success with his, Dave kindly gave me some hints and tips regarding his construction but although I managed to get mine to work much better I was still left with some serious shortcomings. After many hours spent trying to tame the really simple design I read a mail from a fellow LF/MF enthusiast regarding the use of the G0MRF 136 kHz PA on 500 kHz and decided to go that route, see May 2008.
April 2007 I have now evaluated the antenna described previously and found that it has not performed any better than other antenna experiments. The situation as researched in various LF-MF antenna technical articles suggest that although it is necessary to have top capacity with a short vertical antenna for frequency range it will be detrimental to performance if the top-load capacity hat is brought down near to the ground therefore dramatically reducing the effective height of the antenna and thus reducing its performance. So due to my home only having a small area of garden another method will have to be used in order for me to continue to participate in LF and MF experiments. During my research I discovered that if a short LF-MF antenna had a multi-wire vertical section that the capacity will be increased by a large percentage over a single wire vertical, the minimum practical arrangement was to have a 4 wire cage with the wires spaced no less than 10 inches (250mm) to obtain a high increase in capacity. My construction is a 4 wire cage vertical on two 20 inch (510mm) plastic support rings with the wires being about 14 inches (360mm) spaced, in theory using a 12 metre vertical cage would produce an increase in capacity of 140%. I constructed the antenna with about a 10 metre vertical cage, no additional top-load and in practice it only produced a capacity of 275pf compared to 370pf with the previously described antenna. The performance on receive appears to be better. The ultimate test will be to transmit.
Performance on transmit was promising except for one problem, the slightest breeze caused the 4 wires to move around and make large changes to antenna properties and fluctuations to transmitter settings which is undesirable. Therefore if I was able to easily to stabilise the 4 cage wires and add a capacity top hat it could have proved to be a good antenna. Unable to carry that out I am reverting to the previous vertical section of made up of 8 insulated stranded wires twisted together like Litz wire which was very stable and add a new top load.